It is the nature of soldering processes that a flux is necessary for the solder material to wet to a substrate. The flux reacts with and thereby removes oxide surface layers on both the solder and the substrates. This ensures that clean metals are presented during reflow so wetting and associated intermetallic formation can proceed.
Fluxes are generally provided as liquids that can be painted, sprayed or otherwise dispensed onto the metallic surfaces prior to reflow. Also, such liquid fluxes can be used to pre-coat metal surfaces. In this case, the flux is deposited onto the metal and dried prior to use. This approach is often adopted for pre-forms.